Solar storms can throw out communications systems and electricity transmission with high energy particles, but nothing we have seen in the modern era remotely resembles an event from 660 BCE that left its legacy in Greenland Ice. oobqoo/Shutterstock

Storms on the Sun can pose a hazard for us on Earth, but measuring the threat is challenging. The legacy of a storm at least 10 times the size of any we have directly measured has been found in a Greenland ice core, and it's shaking up the idea of how we carry out assessing this risk.

In 1989 a large solar event crashed Quebec's electricity grid and almost took down the US East Coast. Communication systems have become more vulnerable since then. Although efforts are underway to prepare for something like this again, the current risk assessment is based on direct observations made over the last 70 years, and largely ignores the threat of the possibility of something much worse.

Solar storms are high-energy particles unleashed by explosions on the Sun's surface. Earth is constantly being bombarded by particles, but when a solar storm joins the fray, it is much more intense. When high energy particles hit the atmosphere they can produce isotopes such as carbon-14 and beryllium-10 that are rare in other circumstances. These then get trapped in tree rings or ice sheets, and we can determine the approximate timing of their production.

Muscheler was alerted to the possibility of a large solar storm around 2,700 years ago through a rise in carbon-14 in Greenland's ice deposited around this time. On its own, a carbon-14 spike could have other explanations, so Muscheler looked for something more distinctive, and found it in the beryllium-10, confirmed by a chlorine-36 spike at the same time.

Muscheler was also part of a team that identified two other events so powerful they left a detectible legacy of isotopes in ice cores and tree rings, one from 775 CE, the other from 994 CE.

Events like these are usually measured by the number of high energy protons. The largest storm we have been able to study directly occurred in 1956, but Muscheler's Iron Age discovery appears to have had 11 times as many mid-range protons, and 20 times as many high energy protons as that one. These suggest it was more powerful than the 994 CE tempest. It produced fewer protons than the phenomenal event of 775 CE, but was more skewed towards higher energy protons.

The storm no doubt gave people at northern latitudes an auroral display the likes of which they had never seen. However, "If that solar storm had occurred today, it could have had severe effects on our high-tech society," Muscheler said in a statement. "That's why we must increase society's protection again solar storms.”

Having had three such events in 2,700 years, smaller events – drastic enough to crash electricity grids and communication networks, but not be recorded in ice – are probably far more common. Focusing on just 70 years of data can blind us to the danger.

"Our research suggests that the risks are currently underestimated. We need to be better prepared," concluded Muscheler.